Thermal recovery method for oil sands



Oct. 10, 1967 L. K. STRANGE ETAL 3,346,048

THERMAL RECOVERY- METHOD FOR OIL SANDS Filed Dec. 17. 1964 LLOYD K.STRANGE JAMES E. MARBERRY DAVID S. KOONS INVENTORS BY m ew' ATT'ORNEYUnited States Patent THERMAL RECOVERY METHOD FOR OIL SANDS Lloyd K.Strange, Grand Prairie, and James E. Marberry and David S. Koons,Dallas, Tex., assignors to Mobil Oil Corporation, a corporation of NewYork Filed Dec. 17, 1964, Ser. No. 419,035

13 Claims. ((11. 166-41) ABSTRACT OF THE DISCLOSURE carbons arerecovered from an adjacent production well.

The notch is of a longitudinal dimension sufiicient to accommodate thelongitudinal expansion and movement of the conduit within the well whensubjected to fracturing and thermal stresses. The notch may be placedadjacent geological disconformities in the oil sands to control fractureplacement at a desired depth. Logging techniques may be employed tofacilitate further the placement of the notch.

This invention relates to a method for recovering hydrocarbons fromformations within the earth, and more particularly, to a thermalrecovery method for Winning hydrocarbons from oil sands.

There are vast deposits in the earth which contain large amounts ofhydrocarbons and in which, at the present, the hydrocarbons may beclassified as unrecoverable for the lack of a practical and efiicientproducing method. Among these deposits are the oil sands which contain ahighly viscous crude hydrocarbon material not recoverable in its naturalstate through a well by ordinary production methods. The oil sands foundin the deposits along the Athabasca River in Alberta Province, Canada,which are commonly known as the Athabasca tar sands, are a prime exampleof oil sands formations containing hydrocarbons unrecoverable byconventional petroleum producing methods at the present time. Anotherexample is the shallow sandstonelike formations found in Californiawhich contain such viscous crude oils that their recovery byconventional well production methods is unsatisfactory. The hydrocarbonsin these oil sands have great viscosities at ambient formationtemperatures but advantageously undergo a substantial reduction inviscosity when subjected to elevated temperatures. For this reason,thermal recovery procedures appear to bear great promise as a practicaland an economical method for winning these hydrocarbons from oil sands.

Thermal recovery methods generally employ a fluid capable of applyingheat to the immobile hydrocarbons so as to reduce their viscosity. Thisfluid is, of course, passed from the earths surface into the oil sands.These fluids may effect in situ combustion with air being the best knownmaterial for this purpose. Alternatively, the fluids may be priorlyheated, such as steam, and introduced singly or in combination withchemical additives to foster the movement in the oil sands of the heatedhydrocarbons. One problem encountered in thermal recovery methods is theoftentimes low permeability of the 3,346,fl48 Patented Oct. 10, 1967 oilsands to air and other fluids. One solution to this problem is to createa fracture in the oil sands disposed between spaced-apart wells in whichthe fluid for heating the hydrocarbons may be circulated freely throughthe oil sands. However, additional problems are encountered in creatinga fracture in the oil sands.

The oil sands, such as in the Athabasca deposits, have a plastic naturewhich results in a hybrid stress behavior resembling that of a liquidhydrocarbon and a solid substratum. Thus, upon subjecting the oil sandsto hydrostatic fluid pressure, the oil sands yield to redistributepartially the stresses prior to their initial failure in fracturing.This characteristic can result in rupture of the bond between a wellcasing, cement, and the adjacent oil sands. As a result, fluids canbypass portions of the oil sands by traveling along the surfaces partedby such bond rupture. Additionally, placing a fracture at a desireddepth within the oil sands is complicated by the passage of thefracturing fluids along these surfaces in cased wells. Thus, afracturing fluid may pass along such surfaces, such as between thecemented well and the presented face of the oil sands, to some depthother than the desired depth until a suflicient weakness in the oilsands is encountered in which the fracture may be generated. Obviously,this problem of creating a fracture at a desired depth within the oilsands is further complicated by a haphazard assortment of materialsvariable in hydrocarbon content and geological characteristics formingthese formations.

For example, the oil sands in deposits along the Athabasca River arevery variable along their vertical extent in the amounts ofhydyrocarbons they contain. In the oil sands thin beds of clay, silt,and shale sections (including shaley sand) as well as lenses of barrensiltstone and the like are interbedded. Further, these interbeddings maybe continuous or discontinuous in the horizontal depending upon thelocality of the deposit.

As a result of the tendency of fracturing fluids to break the bondbetween a cemented well and the presented face of the oil sands, and forother reasons, it is desired to have good bonding of cement to the oilsands and metal structures. This leads to additional problems when thecemented wells are used for thermal recovery methods. Using a cementedwell for hydraulically fracturing the oil sands places the well conduitsunder severe tension. This can result in breaking the cemented wellbonding to the oil sands, or parting of the conduits cemented in thewell. Thermal stress can also create undesired results. For example,operation of heaters in these cemented wells adjacent perforationsproviding fluid communication to the oil sands results in severe thermalexpansion particularly in the metal conduits in the cemented well. Thisthermal expansion can produce disastrous effects. The liner or metalconduit collapses from thermal distortion and can cause destruction ofany equipment contained Within the cemented well. Additionally oralternatively, the thermal expansion breaks the cement to oil sands bondwith the liner or metal conduit and surrounding cement being extended inthe vertical within the well. Even on shallow wells used for in situcombustion, the liner or metal conduit with its surrounding cement maybe extended two feet above its initial position relative to the earthssurface as a result of thermal stress.

It is the purpose and principal object of this invention to provide athermal recovery method employed for re covering hydro-carbons from asubterranean formation, especially oil sands, without suffering theundesired aforestated results. Another object is to provide a thermalrecovery method for employment in oil sands to produce hydrocarbonswhich resolves the problems heretofore stated. Another object is tocreate expeditiously, even with cemented wells, a fracture in the oilsands adjacent the depth at which it is desired. Another object is tocreate a fracture in oil sands with the rupturing of the bonding of acemented well to the oil sand and the traveling of the fracturing fluidvertically adjacent the cemented well reduced to a minimum. Anotherobject is to avoid the thermal expansion problems suffered when cementedwells are employed in thermal recovery methods practiced in oil sands.Another object is to employ the variable geologi-cal nature of oil sandsto advantage in a thermal recovery method. Another object is to create afracture at an exact depth in the oil sands. Another object is toprovide in a method for the thermal recovery of hydrocarbons thecreation of a fracture adjacent a desired depth in the oil sands andavoidance of undesired thermal distortion effects to cemented wells.Another object is the creation of a fracture in oil sands withfracturing pressures less than previously required. These and otherobjects will be more apparent when read in conjunction with thefollowing detailed description of embodiments of the present invention,the appended claims, and the attached drawings, wherein:

FIGURE 1 is a vertical section taken through the earth illustrating anoil sands formation with the structures employed for carrying out athermal method for recovering hydrocarbons according to the presentinvention;

FIGURE 2 is an enlargement of the left-hand portion of FIGURE 1 but withthe addition of certain structures for locating initiated fractures; and

FIGURE 3 is an enlarged section of FIGURE 1 like FIGURE 2 butillustrating an application of the present method to oil sandscontaining shale sections.

In the drawings and the following description, like structures will bedesignated with like nomenclature and numerals.

Referring to FIGURE 1 of the drawing, a description of the preferredembodiment of the thermal recovery method of this invention will bedescribed in reference to an illustrative subterranean formation. Asubterranean formation 11 is shown which contains a deposit of oil sands12 disposed below the earths surface 13 beneath an overburden 14. Theoil sands 12 rest upon a bedding 16. Assuming the formation 11 to be aportion of the Athabasca River deposit, the overburden 14 usually is athin layer of soil mantle and glacial drift together with other types ofsimilar strata. Also there may be soft sandstones, siltstones, and shalefound in the overburden 14. The overburden 14 is variable in thickness.The bedding 16 in the Athabasca River deposit usually is limestone. Theoil sands 12 are variable in thickness, for example, from several feetto several hundred feet in thickness, and in composition, for example intheir hydrocarbon content. The oil sands 12 contain thin interbeds ofclay 17, silt 18, as well as lenses 19 of barren siltstone and the like.Also interbedded may be one or more stratum of shale which will beidentified and discussed hereinafter. It is to be noted that thesevarious interbeds are usually discontinuous. Sometimes they may beconsidered only to be portions of the oil sands 12 barren of anyhydrocarbons. Although the formation 11 has been particularly describedwith oil sands 12 being Within the Athabasca River deposit, it isobvious that other oil sands deposits are similar in structure.

A plurality of spaced-apart boreholes 23 and 24 are provided extendingfrom the earths surface 13 downwardly to a desired depth into the oilsands 12. Preferably, the boreholes 23 and 24 extend substantially theentire thickness of the oil sands 12. Within the borehole 23 is disposeda metal casing 26 to provide a conduit for conveying fluids between theearths surface 13 and the oil sands 12. The casing 26 is cemented withinthe borehole 23 by a surrounding sheath 27 of cement. The sheath 27 isbonded to the casing 26 and also to the presented face of the oil sands12. The sheath 27 may be formed in any suitable manner. For example,neat cement may be disposed within the casing 26 and then displaceddownward- 1y by means of a cementing plug 28. The neat cement is forcedinto the annulus between the borehole 23 and the exterior of the casing26 as is shown in FIGURE 1.

Additional structures are operably associated with the casing 26 tofacilitate the practice of the present method. An inlet 29 for passingfluids is provided in the casing 26. A Wellhead 31 is secured at the topof the casing 26. The Wellhead 31 is adapted to permit the insertion ofvarious apparatus into the interior of the casing 26. It may be providedwith additional inlet connections for passing fluids to the interior ofthe casing 26, if desired.

The borehole 24 is provided with means for producing formation fluidsfrom the oil sands 12 to the earths surface 13. Such means may take anyform and there are many known structures suited for this purpose. Forexample, the borehole 24 receives a production casing 36 which isprovided with a plurality of perforations 37 along its extent adjacentthe oil sands 12. Preferably, the perforations 37 extend to thelowermost extremities of the borehole 24. The casing 36 is sealed abovethe oil sands 12 to the overburden 14 by any suitable packer means, suchas by a cement and packer assembly 38. The casing 36' carries a wellhead41 on its upper extremity with a tubing 42 concentrically extendingthrough the wellhead 41 down into the lower extremity of the casing 36.If sufficient bottomhole pressure obtains, fluids produced from theformation 12 may be flowed to the surface, or the tubing 42 may beequipped With a suitable downhole pump, not shown. Of course, theborehole 24 can be provided with other types of means for producingformation fluids from the oil sands 12. For example, an inlet 39 may beemployed for the fluid lifting of hydrocarbons through the tubing 42 tothe earths surface 13. The produced fluids can be collected at theearths surface 13 in any suitable type of receiver, which receiver isnot shown in FIGURE 1. However, those skilled in the art can provide asuitable receiver for this purpose.

Although the preceding procedural steps have been disclosed in certainorder, it is recognized that these steps may be carried out in othersequences as full equivalence within the spirit of the present inventionto obtain satisfactory results.

The casing 26 with the surrounding sheath 27, as one unitary and rigidstructure, are bonded over an extensive distance to the oil sands 12 andthe overburden 14. This bonding, of course, provides a necessaryfluid-tight seal to prevent fluids from escaping between the meetingsurfaces of the casing 26, the sheath 27, and the oil sands 12. However,vertical movement between the casing 26 and sheath 27 relative to theadjacent oil sands 12 can produce many undesired results, as has beenearlier described, durmg fracturing and in response to thermalexpansion.

This bonding of the casing 26 with the sheath 27 and to the oil sands 12is preserved by this invention during subsequent fracturing of the oilsands 12. A transverse clrcumferential segment is removed from thecasing 26 and the sheath 27 to produce a window 46 through which fillldSmay easily pass between the casing 26 and the oil sands 12. Preferably,the window 46 is positioned at or ad acent the depth in the oil sands 12at which a fracture is desired to be produced. It will be apparent thatproducing the window 46 severs the casing 26 and sheath 27. With thecomplete separation of the casing 26 and sheath 27, fracturing the oilsands 12 adjacent the depth of the window 46 can produce considerablevertical movements without rupturing the bonding between the oil sands12, the sheath 27, or the casing 26. Obviously, the casing 26 with thesheath 27 can move with the oil sands 12 above the window 46.

Preferably, the window 46 has a longitudinal or vertical dimension atleast equal to the greatest longitudinal dimensional increase of thecasing 26 and sheath 27 resulting from subsequent thermal expansion whenthe oil sands 12 are subjected to the remainder of the thermal recoverymethod. Any means may be employed for producing the window 46. Forexample, the notching equipment shown in US. Patent 3,050,122 may beused for this purpose. With the window 46 having the stated verticaldimension, it will also be apparent'that space is provided in the casing26 and sheath 27 to accommodate elongations caused by subsequent thermalexpansion. Thus, when heated fluids are transmitted via the window 46between the casing 26 and the oil sands 12, the thermal expansion forcescannot cause the casing 26 with the affixed sheath 27 to buckle, orotherwise impair their operability.

The oil sands 12 are fractured by introducing a fluid under pressurefrom the casing 26 through the window 46 into the oil sands 12.Generally, the fluid must be under a pressure initially much greaterthan required merely to raise the overburden 14 and the superimposedpart of the oil sands 12 as a result of the plasticlike nature of theoil sands 12. After a fracture 47 is initiated in the oil sands 12, alesser fracturing pressure will extend this fracture 47 from theborehole 23 toward, and, preferably, to the borehole 24. This pressureis about that amount required to lift the overburden 14 and superimposedoil sands 12. Any fluid may be used under pressures suitable forcreating the fracture 47. Many known fluids are employed in conventionalfracturing procedures and these fluids may be used for producing thefracture 47. For example, the fracturing fluid may be Water with variousadditives such as agents to correct for undesired fluid-loss andnon-Newtonian behavior. Also, the fracturing fluid may include variouspropping agents. For example, it has been found useful in a waterbasedfracturing fluid to add several pounds per gallon of frac sand,preferably in the 20-40 mesh size. The fracture 47 obviously provides apermeable channel through which the fluids employed for the thermalrecovery of hydrocarbons from the oil sands 12 may readily pass betweenthe boreholes 23 and 24.

It is desirable in many instances that the fracture 47 be disposedsubstantially in a horizontal plane. For this purpose, there is formedthrough the window 46 a continuous circumferential V-shaped notch 48into the oil sands 12. The equipment described for producing the Window46 may also be used to produce the notch 48. The notch 48 is oriented ina substantially horizontal plane and needs to penetrate the oil sands 12only a short distance. It has been found that employment of the notch 48with the above-described hydraulic fracturing step produces the fracture47 in a substantially horizontal plane between the boreholes 23 and 24.It is found that less pressureisrequired for the fracturing fluid toinitiate the fracture 47 where the notch 48 is present in the oil sands12. The advantages of such a horizontal fracture 47 are self-evident andinclude increased horizontal sweep efliciency for the thermal recoveryof hydrocarbons from the oil sands 12. Also, a horizontal fracture 47allows freer vertical movement of the superimposed oil sands 12 andoverburden 14. Of course, the notch 48 is placed most nearly adjacentthe depth at which the fracture 47 is desired.

In many instances it will be found that the fracture 47 is not initiatedat a desired depth as a result of the variable nature of the hydrocarboncontent and composition of the oil sands 12. With reference to FIGURE 2,such fracture initiations at undesired depths are illustrated. Thefracturing fluid can break the bond between the sheath 27 and thepresented face of the oil sands 12 to travel to a depth at which astructural weakness of one nature or another exists in the oil sands 12.For example, upon introduction of fracturing fluid through the window46, a fracture 47 is initiated at a lesser depth than desired for thefracture 47. Continued injection of the fracturing fluid under thesecircumstances would result in extension of the fracture 47' at a depthother than the one desired. Under such conditions, a fracture can becreated at a desired depth by the following steps of the present method.First, the depth at which the fracture 47' is initiated is determined.Any suitable means may be employed for this determination. One meansfound well suited for this purpose is to provide in the fracturing fluida radioactive isotope which emits radiation detectable by a suitablegamma-ray detector 51 moved coaxially within the casing 26 by a suitablemechanism. In conjunction with the detector 51 is used a surface readoutmeans 52 for displaying radiation intensities. Movement of the detector51 within the casing 26 will detect a peak radiation intensity at thedepth containing the initiated fracture 47'. Obviously, the casing 26 iscleared of fracturing fluid. Hydraulic cement or other fracture sealingmaterial is injected through the window 46 to fill the initiatedfracture 47. After sealing the fracture 47, fracturing fluid is againinjected through the window 46. Assuming formation of another initiatedfracture 47" to be at a depth other than desired, the steps of detectionand sealing are repeated. Thus, initiated fractures at depths in the oilsands 12 other than that depth desired for the fracture 47 are sealed.The fracture 47 will now be initiated either exactly at, or acceptablyclose to, the depth desired for the fracture 47 in carrying out thethermal recovery of hydrocarbons from the oil' sands 12. The advantagesof such fracture placement can be appreciated when the conditionssurrounding the thermal recovery of hydrocarbons are examined. Oneparticular advantage is in the employment of a fracture disposed withinthe lower third of the oil sands 12 for improving the vertical extent offormation heating. Other advantages will be apparent to those skilled inthe art.

It has been found that in many oil sands 12, besides the interbeddedcl-ays 17, silts 18, lenses of siltstone 19, and the like, there areint-rastratified shale sections 56 as shown in FIGURE 3. The shalesections 56, including shaley sand sections, are varied in nature and ofrandom distribution like the other described structures in the oil sands12. For example, in one deposit of oil sands 12, the shale sections 56are found varied in spacings from one another of 1 foot up to 50 feetwith a thickness between a few inches and 10 feet. It has been found tobe of great advantage in placing the window 46, and the notch 48,adjacent to the shale section 56 nearest to the depth at which thefracture is desired. Subsequent introduction of fracturing fluid underpressure through the window 46 will result in the fracture 47 beingformed along the upper boundary of the lowermost illustrated shalesection 56. Reference to FIGURE 3 will make clear such structure.Although the window 46, and the notch 48, may be some What spaced fromthe nearest shale section 56, the fracture usually will be initiatedalong the upper boundary of the shale section 56. If not, then theinitiated fracture may be detected and sealed as previously described.Then the fracture 47 can be formed where desired.

Preferably, the apex of the notch 48 is placed in horizontal alignmentwith the upper boundary of the shale section 56 nearest to the depth atwhich the fracture is desired. As a result, the fracture 47 is initiatedalong the upper boundary of the shale section 56. If the shale section56 extends between the boreholes 23 and 24, the fracture 47 will followsuch upper boundary to the borehole 24. However, the shale section 56need not be continuous between the boreholes 23 and 24. It is sufiicientwhen it extends only a short distance from the borehole 23 in order thatthe initiation of the fracture 47 occurs in the manner described. Oncethe fracture 47 is initiated, it may be extended in the horizontal ashas been described without a continuous upper boundary of the shalesection 5'6 to follow.

Each shale section 56 may be located in any suitable manner. Forexample, the borehole 23 may be continuously cored through its entiretywhile being drilled into the oil sands 12. The resulting cores disclosethe presence, depth andthickness of each shale section 56 within the oilsands 12. If desired, a conventional resistivity log may be taken aftercompletion of the borehole 23 to disclose the occurrence of each shalesection 56 with a correlation to their depths within the oil sands 12.Other logging methods applicable to open boreholes, of course, may beused. The casing 26 and sheath 27 may be secured within the borehole 23and a cemented well logging method can be used. In this situation, thelocation of each shale section 56 may be determined by natural gamma raylogging. For this purpose, a detector 57 is disposed within the casing26 and arranged for vertical movement therein with a connection to areadout appartus 58 where each shale section 56 is indicated by anincrease in natural gamma ray radiation. Thus, the borehole 23 may belogged by any one of several methods for the presence of one or more ofthe shale section 56 within the oil sands 12.

With the fracture 47 extending between the boreholes 23 and 24, thermalrecovery of the hydrocarbons from the oil sands 12 may be effected. Anysuitable thermal recovery procedure may now be employed to eflect therelease and recovery of the hydrocarbons. For example, such proceduremay include the passing of a fluid under thermal recovery conditionsinto the fracture 47 via the window 46. As a result of heating thehydrocarbons within the oil sands 12, fluids are produced throughperforations 37 into the production casing 36. Thereafter, these fluidsare lifted through the tubing 42 to the earths surface 13. Hydrocarbonsare then recovered from such fluids by a suitable means which may beseletced from means well known to those skilled in the art.

The fluid employed in the thermal recovery procedure, which fluid passesthrough the window 46 into the fracture 47, is preferably a fluid whichobtains in situ combustion in the oil sands 12 when introduced undercombustion conditions into the farcture 47. As an illustration, air, orother combustion-supporting fluids, is forced into the casing 26 throughthe inlet 29. An electric heater 61 is positioned within the casing 26,preferably with its lower extremity slightly above the window 46. Theheater 61 is energized by any suitable means such as by connectionthrough an electrical conductor 62 which extends through the wellhead 31to a surface-disposed generator 63. The generator 63 powered by anysuitable prime mover produces electrical energy which the heater 61converts into heat energy. The heat energy is transferred from theheater 61 to the air flowing into the fracture 47. After a suitableperiod of time, the air has heated the oil sands 12 adjacent theborehole 23 sufiicently to initiate combustion along the fracture 47. Atthis time the heater 61 may be deenergized since combustion within thefracture 47 is self-sustaining upon the continued injection of air. Theinjection of air into the fracture 47 is continued until a suitableproduction of hydrocarbons has been recovered from the oil sands 12.

A heated fluid, which is passed through the window 46 into the facture47, may 'be used in the thermal recovery procedure, if desired.Preferably, the heated fluid is steam, with or without variousadditives, such as caustic. Of course, the borehole 23 is provided withsuitable equipment to facilitate the injection of the heated fluid.

Although several examples of fluids to effect the thermal recovery ofhydrocarbons have been described, other fluids may be employed. Forexample, liquids may be passed through the fracture 47 under thermalrecovery conditions for making mobile the hydrocarbons contained in theoil sands 12.

It has been found that the notch 48 at the window 46 produces in the oilsands 12 not only fractures oriented in the horizontal but fractureswhich have a predominant directional characteristic of propagation inthe horizontal. Thus, once this characteristic is known for any oilsands 12, it is preferred to orient the boreholes 23 and 24 from oneanother in alignment with the predominant direction of propagation ofthe hydraulically induced fracture 47 in the oil sands 12. By thismeans, greater horizontal extension of the fracture 47 may be obtainedwith the expectations that the amount of energy for effecting hydraulicfracturing is at a minimum for each unit area of the fracture 47 inducedinto the oil sands 12.

It will be obvious that the steps for providing the fracture 47 in theforegoing embodiments of the present method are of great utility in andof themselves. Also, these steps can be utilized in any method forthermally recovering hydrocarbons from a subterranean formation by theplacement of the fracture 47 at a desired depth. As one example, thesesteps can be utilized to great advantage where only the borehole 23 isemployed, as has been described, without using the borehole 24 as themeans for producing formation fluids. For this purpose, the injection ofthe fluid used in the thermal recovery procedure through the window 46is interrupted while the resultant formation fluids are produced fromthe oil sands 12 via the fracture 47 into the casing 26 and then, fromwhich fluids, are recovered hydrocarbons.

From the foregoing it will be apparent that a method is disclosed whichaccomplishes all of the stated objects of this invention. Variousmodifications of the disclosed method may be made by those skilled inthe art without departing from the spirit of this invention. Similarly,the disclosed steps of this method when employed both in combination andin sub-combination are of utility. For this and other reasons, thepresent description is intended to be illustrative of this invention,and only the appended claims are to be considered as limitative of theinvention.

What is claimed is:

1. A method for the thermal recovery of hydrocarbons from a deposit ofoil sands residing in a subterranean formation, said oil sands havingthin interbeds of clay and silt, shale sections and siltstone lenses,comprising the steps:

(a) providing a plurality of spaced-apart boreholes in the formationwhich extend into the oil sands,

(b) cementing to the surrounding formation and oil sands, within a firstholehole, a conduit for conv'eying fluids,

(c) providing a second borehole with means for producing formationfluids therefrom,

(d) removing from the conduit and surrounding cement a transversecircumferential segment producing a window therein having a verticaldimension at least equal to the longitudinal dimensional increase of theconduit and surrounding cement resulting from subsequent thermalexpansion, said window produced at about the depth in the oil sandswhere a fracture is to be initiated,

(e) introducing a fluid under pressure into the oil sands from theconduit via the window to create a fracture in the oil sands extendingfrom the first borehole in the direction of the second borehole,

(f) passing a fluid under thermal recovery conditions into the fracturevia the window,

(g) producing fluids from the second borehole, and

(h) recovering hydrocarbons from such fluids.

2. The method of claim 1 wherein the first and second boreholes arealigned with the direction of preferential propagation of the inducedfractures in the oil sands.

3. A method for the thermal recovery of hydrocarbons from a deposit ofoil sands residing in a subterranean formation, said oil sands havingthin interbeds of clay and silt, shale sections and siltstone lenses,comprising a the steps:

(a) providing a plurality of spaced-apart boreholes in the formationwhich extend into the oil sands,

(b) cementing to the surrounding formation and oil sands, within a firstborehole, a conduit for conveying fluids,

(c) providing a second borehole with means for producing formationfluids therefrom,

(d) removing from the conduit and surrounding cement a transversecircumferential segment producing a window therein having a verticaldimension at least equal to the longitudinal dimensional increase of theconduit and surrounding cement resulting from subsequent thermalexpansion, said window produced at about the depth in the oil sandswhere a fracture is to be initiated,

(e) forming into the oil sands at the window a continuouscircumferential V-shaped notch with such notch oriented in asubstantially horizontal plane,

(f) introducing a fluid under pressureinto the oil sands from theconduit via the window to create a fracture in the oil sands extendingfrom the first borehole in the direction of the second borehole,

(g) passing a fluid under thermal recovery conditions into the fracturevia the window,

(h) producing fluids from the second borehole, and

(i) recovering hydrocarbons from such fluids.

4. The method of claim 3' wherein the apex of said V-shaped notch isdisposed in the oil sands adjacent a shale section nearest to the depthat which a fracture is desired to be initiated.

5. The method of claim 3 wherein the apex of said V- shaped notch ispositioned in horizontal alignment with the upper boundary of a shalesection, which boundary is nearest to the depth at which a fracture isdesired.

6. A method for the thermal recovery of hydrocarbons from a deposit ofoil sands residing in a subterranean formation, said oil sands havingthin interbeds of clay and silt, shale sections and siltstone lenses,comprising the steps:

(a) providing a plurality of spaced-apart boreholes in the formationwhich extend into the oil sands,

(b) cementing to the surrounding formation and oil sands, within a firstborehole, a conduit for conveying fluids,

(c) providing a second borehole with means for producing formationfluids therefrom,

(d) removing from the conduit and surrounding cement a transversecircumferential segement producing a window therein having a verticaldimension at least equal to the longitudinal dimensional increase of theconduit and surrounding cement resulting from subsequent thermalexpansion, said window positioned at about the depth in the oil sandsWhere a fracture is to be intiated,

(e) introducing a fluid under pressure into the oil sands via the windowto initiate a fracture,

(f) determining the depth in the oil sands at which the fracture isinitiated,

(g) sealing each initiated fracture at a depth other than at the desireddepth,

(h) repeating steps (e), (f), and (g) until a fracture is initiated inthe oil sands at the desired depth,

(i) continuing the introduction of a fluid under pressure to extend thefracture in the oil sands at the desired depth from the first boreholein the direction of the second borehole,

(j) passing a fluid under thermal recovery conditions into the fracturevia the window,

(k) producing fluids from the second borehole, and

(l) recovering hydrocarbons from such fluids.

7. A method for the thermal recovery of hydrocarbons from a deposit ofoil sands residing in a subterranean formation, said oil sands havingthin interbeds of clay and silt, shale sections and siltstone lenses,comprising the steps:

(a) providing a plurality of spaced-apart boreholes in the formationwhich extend into the oil sands,

(b) cementing to the surrounding formation and oil sands, within a firstborehole, a conduit for conveying fluids,

(c) providing a second borehole with means for producing formationfluids therefrom,

(d) removing from the conduit and surrounding cement a transversecircumferential segment producing a window therein having a verticaldimension at least equal to the longitudinal dimensional increase of theconduit and surrounding cement resulting from subsequent thermalexpansion, said window positioned in the oil sands at about the depth atwhich a fracture is to be initiated,

(e) forming into the oil sands at the window a continuouscircumferential V-shaped n-ot-ch with such notch oriented in asubstantially horizontal plane,

(f) introducing a fluid under pressure into the oil sands via the windowto initiate a fracture,

(g) determining the depth in the oil sands at which the fracture isinitiated,

(h) sealing each initiated fracture at a depth other than the desireddepth,

(i) repeating steps (f), (g), and (h) until a fracture is initiated inthe oil sands at the desired depth,

(j) continuing the introduction of a fluid under pressure to extend thefracture in the oil sands at the desired depth from the first boreholein the direction of the second borehole,

(k) passing a fluid under thermal recovery conditions into the fracturevia the window,

(1) producing fluids from the second borehole, and

(rn) recovering hydrocarbons from such fluids.

8. A method for the thermal recovery of hydrocarbons from a deposit ofoil sands residing in a subterranean formation, said oil sands havingthin interbeds of clay and slit, shale sections and siltstone lenses,comprising the steps: a

(a) providing a plurality of spaced-apart boreholes in the formationwhich extend into the oil sands,

(b) logging a first borehole to determine the depth of shale sections inthe vicinity of the depth in the oil sands at which a fracture isdesired to be initiated,

(c) cementing to the surrounding formation and oil sands, within a firstborehole, a conduit for conveying fluids,

((1) providing a second borehole with means for producing formationfluids therefrom,

(e) removing from the conduit and surrounding cement a transversecircumferential segment producing a window therein having a verticaldimension at least equal to the longitudinal dimensional increase of theconduit and surrounding cement resulting from subsequent thermalexpansion, said window provided at about the depth in the oil sands of ashale section nearest to the depth where a fracture is to be initiated,

(f) forming into the oil sands at the window a continuouscircumferential'V-shaped notch with such notch oriented in asubstantially horizontal plane and 'With the apex of the notchpositioned adjacent the shale section nearest to the depth at which afracture is desired to be initiated,

(g) introducing a fluid under pressure into the oil sands from theconduit via the window to create a fracture in the oil sands extendingfrom the first borehole in the direction -of the second borehole,

. (h) passing a fluid under thermal recovery conditions into thefracture via the window,

(i) producing fluids from the second borehole, and

(j) recovering hydrocarbons from such fluids.

9. The method of claim 8 wherein the apex of the notch is positioned inhorizontal alignment with the upper boundary of the shale section.

10. A method for the thermal recovery of hydrocarbons from a deposit ofoil sands residing in a subterranean formation, said oil sands havingthin interbeds of clay and silt, shale sections and siltstone lenses,comprising the steps:

(a) providing a plurality of spaced-apart boreholes in the formationwhich extend into the oil sands,

(b) logging a first borehole to determine the depth of shale sections inthe vicinity of the depth in the oil sands at which a fracture isdesired to be initiated,

(c) cementing to the surrounding formation and oil sands, within a firstborehole, a conduit for conveying fluids,

(d) providing a second borehole with means for producing formationfluids therefrom,

(e) removing from the conduit and surrounding cement a transversecircumferential segment producing a window therein having a verticaldimension at least equal to the longitudinal dimensional increase of theconduit and surrounding cement resulting from subsequent thermalexpansion, said window provided at about the depth in the oil sandsadjacent a shale section nearest to the depth at which a fracture is tobe initiated,

(f) introducing a fluid under pressure into the oil sands from theconduit via the window to create a fracture extending from the firstborehole in the direction of the second borehole,

(g) passing a fluid under thermal recovery conditions into the fracturevia the window,

(h) producing fluids from the second borehole, and

(i) recovering hydrocarbons from such fluids.

11. The method of claim wherein the window is positioned in horizontalalignment with the upper boundary of the shale section.

12. A method for creating a fracture at about a desired depth in adeposit of oil sands residing in a subterranean formation from aborehole subject to thermal expansion, said oil sands having thininterbeds of clay and silt, shale sections and siltstone lenses,comprising the steps:

(a) providing in the formation to extend into the oil sands a boreholeand therein surroundingly cementing a conduit for conveying fluidsbetween the oil sands and the earths surface,

(b) removing from the conduit and surrounding cement a transversecircumferential segment producing a window therein having a verticaldimension at least equal to the longitudinal dimensional increase of theconduit and surrounding cement resulting from subsequent thermalexpansion, said window positioned at about the depth in the oil sandswhere a fracture is to be initiated,

(c) introducing a fluid under pressure into the oil sands via the windowto initiate a fracture,

(d) determining the depth in the oil sands at which the fracture isinitiated,

(e) sealing each initiated fracture at a depth other than at the desireddepth,

(f) repeating steps (c), (d), and (e) until a fracture is initiated inthe oil sands at the desired depth, and

(g) continuing the introduction of a fluid under pressure to extend thefracture at the desired depth from the borehole into the oil sands.

13. A method for creating a fracture at about a desired depth in adeposit of oil sands residing in a subterranean formation from aborehole subject to thermal expansion, said oil sands having thininterbeds of clay and silt, shale sections and siltstone lenses,comprising the steps:

(a) providing in the formation to extend into the oil sands a boreholeand therein surroundingly cementing a conduit for conveying fluidsbetween the oil sands and the earths surface,

(b) removing from the conduit and surrounding cement a transversecircumferential segment producing a window therein having a verticaldimension at least equal to the longitudinal dimensional increase of theconduit and surrounding cement resulting from subsequent thermalexpansion, said window positioned at about the depth in the oil sands atwhich a fracture is to be initiated,

(c) forming into the oil sands at the window a continuouscircumferential V-shaped notch with such notch oriented in asubstantially horizontal plane,

(d) introducing a fluid under pressure into the oil sands via the windowto initiate a fracture,

(e) determining the depth in the oil sands at which the fracture isinitiated,

(f) sealing each initiated fracture at a depth in the oil sands otherthan the desired depth,

(g) repeating steps (d), (e), and (f) until a fracture is initiated inthe oil sands at the desired depth, and

(h) continuing the introduction of a fluid under pres sure to extend thefracture at the desired depth from the borehole into the oil sands.

References Cited UNITED STATES PATENTS 2,368,424 1/1945 Reistle 16642 X3,018,095 1/1962 Redlinger 16642 X 3,050,119 8/1962 Fast 16642 3,145,7728/1964 Huitt 16611 X 3,196,945 7/1965 Craig et a1. 166-11 3,211,22110/1965 Huitt 166-42 CHARLES E. OCONNELL, Primary Examiner.

NILE C. BYERS, JR., Examiner.

13. A METHOD FOR CREATING A FRACTURE AT ABOUT A DESIRED DEPTH IN ADEPOSIT OF OIL SANDS RESIDING IN A SUBTERRANEAN FORMATION FROM ABOREHOLE SUBJECT TO THERMAL EXPANSION,